Needle Ice: nature’s pasta extruder

Temperature Gradients

It’s warm, it’s cold. Then warm again, and now cold. And so it goes with our winters. Temperature gradients exist throughout a forest and so just how warm and how cold it is depends on where you are, whether you’re up in elevation, down in depth, under in snow, or inside in wood:
  • Tree bark is likely to be warmer than the wood during the day, cooler at night
  • Soil temperature is less fickle than the air, and likely to be warmer throughout the winter
  • Animal dens are warmed by the animal’s body and are warmer than air outside the entrance to the burrow
  • Snow packs can have temperature gradients from the top to bottom
  • The summit of Camel’s Hump is cooler than the the bottom
  • The air immediately above Lake Champlain (particularly before it freezes) is often warmer than the air above the surrounding land
Two of my favorite ice formations that appear in the winter as a result of strong temperature gradients are: hoar frost and needle ice.

How It Forms

Soil takes longer to cool than the air does, so in the earlier parts of winter the air may be well below freezing, while the ground is still thawed and water flows freely through it. If the air is sufficiently cold, water at or near the soil surface may freeze. Water expands when it freezes and this ice expands into the empty space above. If the air is cold enough, it continues to freeze water near the surface. The expanding ice pushes ever upwards, drawing more water up through the soil by capillary action. The process continues, with endless tiny spires being thrust through the soil. Above the soil the spires may fuse together, forming small chunks 2-3″ thick.

Sadly, once the soil freezes completely, the process ceases. You”re most likely to encounter needle ice in late fall before the soil freezes and again in early spring as it thaws. It’s also common in more mild winters (and in those odd spells where temperatures soar up to nearly 60). Essentially anytime the upper layers of soil have liquid water and when air temperatures are well below freezing you can find needle ice on a trail near you.

Where It Forms

Soil

Needle ice forms in soils that are somewhat porous, but that are also saturated with water. While sandy soils are certainly porous, water drains right through them. On the other hand, though clay soils retain their moisture, they’re not nearly so porous. Silt soils are the Goldilocks of needle ice soils. Like sand, silt particles are roundish so they don’t adhere to each other as well as clay particles do (clay particles are flat and block the upward/downward mobility of water). Silt particles are much smaller than sand and so water percolates much slower through these soils.

Air

The key to needle ice formation is that the air has to be much cooler than the wet soil. So needle ice tends to form at night, when the air is coolest and the temperature gradient the greatest. It also tends to form on trails where foot traffic breaks down leaf litter and roots, exposing the wet soils to the cold air.

More on the topic

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